Embryology Lecture, #3 Ovulation, Fertilization, and Implantation

What are the 4 stages of follicular development?

1. Primordial follicles contain the immature oocytes

2. Primary follicles begin to mature, granulosa cells proliferate

3. Secondary follicles allow for antrum formation and further maturation

4. The fully mature follicle (Graafian follicle) is ready for ovulation

After ovulation, old granulosa/theca cells form the:

Corpus luteum

If no fertilization occurs, the corpus luteum degenerates to form the:

corpus albicans (scar tissue)

What hormone does the corpus luteum secrete?

1. Progesterone

What are some characteristics of the corpus luteum?

1. Storage site for cholesterol

2. Becomes vascularized

What occurs in the follicular phase of the ovarian cycle?

1. FSH stimulates several follicles to grow

2. One follicle becomes dominant, the rest degenerate

3. Mature follicle secretes lots of estrogen

What days does the follicular phase typically occur in the ovarian cycle?

Days 1-14

On day 14 of the ovarian cycle, what occurs?

Ovulation

What occurs during ovulation of the ovarian cycle?

Rupture of mature follicle and release of egg

What are the steps within the LH/FSH Surge and Ovulation?

1. Increased estrogen for 1-2 days

2. Positive feedback, the LH/SH SURGE

3. The primary oocyte becomes the secondary oocyte

4. ovulation

5. Follicle transformed into the corpus luteum

What occurs during the luteal phase of the ovarian cycle?

1. Corpus luteum secretes lots of progesterone and estrogen to prepare the uterus for pregnancy

If the egg is not fertilized, what occurs to the corpus luteum?

It will degenerate (typically on day 24)

The luteal phase of the ovarian cycle is always how many days long?

15-28 days

What days of the ovarian cycle does the luteal phase occur?

Days 15-28

Negative feedback prevents what from occurring in the luteal phase?

It needs negative feedback so ovulation does not occur again (BASIS OF HORMONAL BIRTH CONTROL)

The basis of hormonal birth control is the presence of progesterone, which does what?

Produces the negative feedback, turning off FSH and LH, so another ovulation does not occur

This hormone is produced by developing follicles, stimulating endometrial thickening

Estrogen

This hormone is triggered by high estrogen levels, causing ovulation

LH surge

This hormone is secreted by the corpus luteum, preparing the endometrium for potential implantation

Progesterone

The cumulus oophorus cells are made from what cells?

Granulosa cells

What are the signs and symptoms of ovulation?

1. Basal body temperature rise (slight increase post-ovulation)

2. Cervical mucus changes (becomes clearer, more stretchy)

3. Mittelschmerz (middle pain)

4. Biochemical indicators (LH in urine)

This is the union of a sperm and an oocyte, resulting in the formation of a zygote, marking the beginning of a new organism with a unique genetic identity

Fertilization

The corona radiata is made up of what cells?

Graulosa cells

this is a biochemical change in sperm that increase motility and ability to penetrate the oocyte

Capacitation

Where does capacitation occur?

The female reproductive tract

What are the 3 phases of fertilization?

1. Penetration of corona radiata

2. Penetration of zona pellucida

3. Fusion of oocyte and sperm cell membranes

Once the spermatozoon enters the cytoplasm of the oocyte, the egg responds in three ways:

1. Cortical and zona pellucida reactions (IMPENETRABLE)

2. Oocyte finishes the second meiotic division (ARRANGE OOCYTE CHROMOSOMES IN A VESICULAR NUCLEUS CALLED THE FEMALE PRONUCLEUS)

3. Metabolic activation of the egg (RELEASE OF CALCIUM IN EGG, INITIATES DEVELOPMENT OF THE ZYGOTE)

While the egg is responding to the spermatozoon entering the egg, how is the sperm responding?

1. Spermatozoon moves forward until it is close to the female pronucleus

2. Its own nucleus becomes swollen and forms the male pronucleus

3. Tail detaches and degenerates

4. Female and male pronuclei are morphologically indistinguishable

5. Will come in close contact and lose their nuclear envelopes

What occurs after the fusion of gametes?

1. Each pronucleus replicates its DNA

2. After DNA synthesis, chromosomes organize on the spindle for normal MITOTIC DIVISION

What occurs in the first mitotic division after fertilization?

Chromosomes split and sister chromatids move to opposite poles

Each cell of the zygote has normal DIPLOID number of chromosomes/DNA

Deep furrow appears on the cell surface, dividing the cytoplasm into two parts

What occurs when the zygote reaches the “two-cell stage”?

1. Undergoes series of mitotic divisions, increasing the number of cells (THE CELLS GET SMALLER), which are blastomeres (rapid cell division without growth)

The cells that are created after the two-cell stage are called:

Bastomeres

16 cells are called the:

Morula and it has inner cell mass and outer cell mass

Explain how implantation BEGINS:

The morula will enter the uterine cavity and fluid begins to penetrate through the zona pellucida, going into the intercellular spaces of the inner cell mass

After the start of implantation, the intercellular spaces become confluent and a single cavity, called the:

Blastocele

The intercellular spaces of the morula become confluent and a single cavity (called the blastocele), but overall it is now called the:

Blastocyst

when does the zygote become the blastocyst?

about 4 and a half days after fertilization

The cells of the inner cell blastocyst are called the:

Embryoblast

The outer cell blastocyst are called the:

Trophoblast

Once this layer has disappeared, implantation can occur:

Zona pellicuda

By day 6 after fertilization, the trophoblast cells:

begin to penetrate between epithelial cells of the uterine mucosa

Cells in the embryoblast will differentiate into what 2 cell types?

Epiblast

Hypoblast

After the epiblast and hypoblast cells become differentiated, what is established?

Dorsal-ventral polarity

Some of the hypoblast cells are specified to form the:

Anterior visceral endoderm

What is the end function of the anterior visceral endoderm?

They will migrate to what will become the cranial end of the embryo

When is the cranial-caudal embryonic axis established?

Near the time of implantation (5.5-6 days)

What occurs in the first week of human development?

1. Oocyte immediately after ovulation

2. Fertilization (12-24 hours after ovulation)

3. Male/female pronuclei

4. Spindle of 1st mitotic division

5. Two-cell stage (about 30ish hours of age)

6. Morula with 12-16 blastomeres (about 3 days)

7. Advanced morula reaching uterine lumen (about 4 days)

8. Early blastocyst (4 and a half days of age)

9. Early phase of implantation (blastocyst about 6 days old)

What are the 3 phages of the uterine cycle?

1. Proliferative phase

2. Secretory phase

3. Menstral phase (NO FERTILIZATION)

What occurs in the proliferative phase of the uterine cycle?

Endometrial thickening post-menstruation

under the influence of estrogen

parallels growth of ovarian follicles

What occurs in the secretory phase of the uterine cycle?

After ovulation

In response to progesterone from corpus luteum, becoming more glandular and vascularized

What occurs in the menstrual phase of the uterine cycle?

Shedding of endometrium

only occurs if no fertilization

If fertilization occurs, what happens in the uterus?

Endometrium assists in the implantation and contributes to the formation of placenta

What are the layers of endometrium after fertilization and implantation?

1. Basal layer

2. Spongy layer

3. Compact layer